C. Kozmutza

An efficient method for using molecular symmetry: The presence of model geometries

Abstract In a series of papers a new method which uses the molecular symmetry for treatment of extended molecules is investigated. In this paper it is shown that in the presence of model geometries the method — on which a new ab initio program named SYCETY is based — is effective for calculation of large, related systems.

DECOMPOSITION OF THE TOTAL ENERGY AT THE HF-SCF LEVEL AND AT SEVERAL LEVELS OF CORRELATION. I. A STUDY OF THE INTERACTION IN CLUSTERS OF HE, NE AND AR ATOMS

The interaction energies between two He atoms, two Ne atoms and two Ar atoms were calculated first at the HF-SCF level by a new decomposition scheme using separated molecular orbitals (SMOs). Special attention was paid to the effect of the counterpoise (CP) method. The results show that the CP procedure is an acceptable device for estimating some basis set deficiencies in the systems studied. When using the SMO-LMBPT scheme, on the other hand, the CP calculations become unnecessary. The correlation energy in some He n and Ne n (n = 2, 3, 4) clusters have been studied using the SMO-LMBPT formalism at the second, third and fourth levels of MBPT. The effect of the CP method on the intracorrelated parts was studied and the conclusions are similar to those obtained at the HF-SCF level. Both the intra- and intercorrelated parts, as expected, show transferability properties in the systems studied.

Decomposition of the interaction correlation energy in terms of localized orbital contributions

The interaction energy between two He atoms was computed via a specific supermolecule method. The essence of the approach lies in using a localized representation both in occupied and in virtual space. This method, localized many-body perturbation theory, makes it possible to investigate the correlation energy contributions in a straightforward manner.

The structure of the CH2O-NH3 system: Part I. Study of configurations

Abstract The interaction energy of CH 2 O and NH 3 was calculated at the SCF level. Various small basis sets were used and the basis-set superposition errors were taken into account. It was found that the SCF interaction energy is small (smaller than that between CH 2 O and H 2 O), and it is comparable in magnitude with the dispersion energy calculated using empirical formulae. Well-defined local minima were found at three different configurations. The largest binding energy was that for a staggered configuration when both CH 2 O and NH 3 molecules are in slightly rotated positions.

Characterization of charge distribution in terms of localized orbitals

Localized orbital densities have been investigated in a series of ten-electron hydrides. It has been found that changes in the central atom nuclear charge cause systematic modifications in the localized electronic structure. The charge distributions of localized orbitals have been analyzed using their electric moments.

The use of MIDI basis set at the correlated level

Abstract The H 2 O + H 2 O, the NH 3 + NH 3 , the BH 3 + H 2 O and the Ne + Ne systems have been studied in the supermolecule approach, using several medium sized basis sets (especially the so-called MIDI basis set). The calculations have been carried out by the use of localized molecular orbitals (LMOs). The dispersion interaction energies have been computed by a new method (Kozmutza and Kapuy; Int. J. Quantum Chem., 38 (1990) 665), whose essence lies in the use of LMO contributions at the correlated level. The method proposed seems to be useful for the H 2 O + H 2 O, the NH 3 + NH 3 , and the Ne + Ne systems at different intermolecular distances, using the MIDI basis, but fails in describing correctly the correlation energy for the BH 3 + H 2 O system.

The structure of the CH2O-NH3 system: Part II. Effect of the basis-set superposition

Abstract This paper is a continuation of previous work on the CH 2 O-NH 3 system. The supermolecule approach is applied by using a minimal type (MINI) basis at both the SCF and the correlated levels. The results have shown that only certain orbitals play a role in the counterpoise SCF energy lowering. Similarly, in the MBPT/2 calculations it was found that the corrections due to the basis set superposition error (BSSE) for the correlation energies contain significant contributions only from certain virtual orbitals.

Dependence on the geometry and on the basis set of localized orbital energy and moment contributions: III. Electric moments

Several regularities have been found for the localized orbital energy contributions in a study by using various basis sets and molecular geometries. The localized charge densities have been further analyzed in terms of its first and second order electric moments. The results obtained for these localized moments of the systems investigated (HF, H2O and NH3, respectively) affirm that there are systematic changes in the localized charge distributions as going from the experimental to the theoretically determined equilibrium geometries.

Dependence on the geometry and on the basis set of localized orbital energy and moment contributions: II. Interaction Energies

The Coulomb (J), the exchange (K) and the total (2J-K) interaction energy contributions between localized orbitals have been studied for molecules HF, H2O, NH3 and CH4, respectively. Different basis sets (one of (sp/s) and another of (spd/s) type) were used for the calculations, which were carried out at the experimental and at the theoretical equilibrium geometries. Several regularities were found which could be related to the results obtained in earlier works for the localizability as well as for the spatial distributions of localized charge densities.

Dependence on the geometry and on the basis set of localized orbital energy and moment contributions

In a series of papers we investigate the localized orbital contributions at the molecular experimental and theoretical equilibrium geometries using various basis sets. The present study deals with some energy quantities obtained from localized charge densities: the kinetic, the (effective) potential and the selfinteraction energies are discussed. Several regularities were found for the systems considered, namely the molecules HF, H2O, NH3 and CH4, respectively.